Nanowires and Fuel Cells

By Nancy Huynh and Jessica Hahne
March 14, 2012 03:55

Wind power and solar power are at the forefront of the current movement toward clean and green energy, but they have one major problem. These renewable energy sources, although environmentally friendly, are not constantly available and thus cannot provide continuous power. Sunlight can be harnessed only during the day, while the presence of wind can vary both temporally and geographically. Instead of focusing on these inconsistent power supplies, researchers like Dr. Andre Taylor, Assistant Professor of Chemical & Environmental Engineering, turn their attention to the potential of fuel cells.

Fuel cells have the potential to constantly produce energy from more reliable renewable sources like hydrogen and alcohols. They are likely to play this central role in converting these energy sources to electricity or heat, because their 60% efficiency is almost twice that of current power plants. Combined heat and power units that take advantage of the heat produced as well can boost this efficiency up to over 70%, sometimes even as high as 90%. Since fuel cells do not rely on the process of combustion but rather on electrochemical redox reactions, they release much lower levels of pollutants than conventional generators do, which further contributes to the idea of achieving clean energy.

How Does a Fuel Cell Work?

A fuel cell, like a battery, consists of a positively charged anode and a negatively charged cathode with an ion-conducting electrolyte in between. While batteries store chemical energy, fuel cells convert chemical energy to electricity or heat. Fuel is added at the anode where an oxidation reaction occurs to split, for example, methanol into hydrogen ions and carbon dioxide. The electrons from the oxidation reaction migrate from the anode toward the cathode, creating a current that can do work. At the cathode, these electrons, hydrogen ions, and oxygen participate in a reduction reaction to produce water.

The net reaction is clean, with methanol and oxygen as the reactants and water and carbon dioxide as products. Although carbon dioxide is considered a greenhouse gas, fuel cells do not yield additional, more harmful pollutants (i.e. NOx). The oxidation-reduction cycle of the fuel cell can continue as long as fuel is provided. Although a single fuel cell may not be able to produce a large voltage, multiple cells them can be combined into stacks to produce more net energy.

The five main types of fuel cells are differentiated by their electrolytes, each of which have different properties. Phosphoric acid fuel cells have phosphoric acid as the mobile ion carrier and works at 100-220°C. Alkaline fuel cells are similar but use KOH. Lower temperature fuel cells like these require pure hydrogen as fuel. In contrast, molten carbonate and solid oxide fuel cells operate at extremely high temperatures of over 600°C, which allows harnessing of their thermal energy and utilization of a wider range of fuels since they do not need a fuel reformer to convert fuels into pure hydrogen. Within the cells, ions pass through an alkali carbonate mixture and solid, nonporous metal oxide, respectively. Finally, polymer electrolyte or proton exchange membrane fuel cells (PEFC) use a polymer membrane that conducts protons. These fuel cells operate at lower temperatures of around 80°C but require pure hydrogen as fuel. Direct alcohol fuel cells (DAFCs), which the Taylor laboratory works with, are a type of PEFC that can use alcohol instead of hydrogen as a fuel source.

Fuel Cell Obstacles

The main challenge in fuel cell research is to improve the efficiency and durability of the catalysts in order to process more fuel. Currently, platinum is the most efficient electrocatalyst for accelerating chemical reactions in fuel cells, but it is also particularly susceptible to carbon monoxide “poisoning” at the lower temperatures at which DAFCs operate. Although the ideal fuel cell reaction produces only carbon dioxide and water, carbon monoxide is produced as an intermediate along the reaction pathway. This chemical can be absorbed on the platinum surface sites, which strongly bind carbon monoxide, inhibiting fresh reactant molecules from participating in further reactions, decreasing the catalyst’s efficiency. Researchers are trying to minimize the extent of this poisoning.

When evaluating the platinum catalyst, however, scientists also want to maximize the catalytic surface area to increase the probability of reactions occurring and produce more electricity. A porous carbon black material is most commonly used as a supported catalyst, to which platinum adsorbs. Yet platinum particles can become trapped in carbon pores where they cannot touch the electrolyte. Without contact among the fuel, electrically connected catalyst regions, and electrolyte together (collectively termed the triple phase boundary), the oxidation products simply recombine because there is no diffusion pathway for electrons and protons. The catalyst then essentially becomes useless; in fact, only 20-30% of the catalyst is utilized at the interface. This inefficient usage of expensive platinum adds up significantly when fuel cells are combined into large stacks.

Carbon nanotubes, another area of research in the Taylor laboratory, circumvent this problem by applying platinum on the external surface of nanotubes; the cylinder’s diameter (2-4 nm) is too small for many of the 2-3 nm particles to become trapped. The nanotubes’ high aspect ratio of length versus diameter also grants them higher conductivity inside a catalyst matrix. However, carbon nanotubes become gradually corroded over many voltage cycles, and the platinum becomes detached or can agglomerate, reducing surface area and productivity. A more durable material is needed to ensure the advancement and further development of carbon nanotubes.

Electron microscopy image of bulk metallic glass nanowires with a molecular schematic of their components and interactions. Courtesy of Dr. Taylor.

Bulk Metallic Glasses as New Catalysts

Bulk metallic glasses (BMGs) are amorphous metal alloys that can be supercooled without crystallizing and thus maintain their amorphous nature, which can be more easily molded into many forms as long as they remain under the crystallization temperature. Metals have a high propensity for crystallization, so their amorphous states are usually achieved by a process of fast freezing. Unfortunately, many would require a cooling rate on the scale of 105-106 Kelvin per second to reach this state, beyond the capabilities of most equipment. In comparison, the alloy that the Taylor laboratory works with, Pt57.5Cu14.7Ni5.3P22.5, can avoid crystallization with a more feasible cooling rate of 102 Kelvin per second.

Research on bulk metallic glasses is actually the work of Jan Schroers, Associate Professor of Mechanical Engineering & Materials Science, also at Yale. However, after being personally invited to his colleague’s research presentation at a conference, Taylor realized the potential of this material when applied to fuel cells and electrochemical devices. Since then, both laboratories have worked as a team with Schroers providing the bulk metallic glasses and Taylor investigating the electrochemical structure property relationships of these materials. Their joint findings were recently featured on the cover of ACS Nano’s April issue.

BMGs are an exciting material to work with in fuel cells because they can be made into many different shapes, such as nanowires, with relatively simple top-down processing (hot-pressing into a mold). The Taylor and Schroers team has already successfully created nanowires 15 nm in diameter, but they are pursuing still smaller tubes to optimize the surface area of the platinum catalyst. Although BMGs may consist of different metals in different ratios, the laboratory has specifically worked with Pt57.5Cu14.7Ni5.3P22.5, partly because it contains the important platinum catalyst. This material has a lower cost than pure platinum but also retains high electrical conductivity, leading to high productivity on the platinum surface.

Does exposing more platinum just bring back the issue of carbon monoxide poisoning? No, because alloys have a bifunctional mechanism solution. One of the other metals in the alloy can be oxophilic and adsorb a hydroxyl group, which can then react with carbon monoxide to produce carbon dioxide and free a catalytic site. With regards to durability, after normalizing for surface area over 1000 cycles, while conventional Pt/C catalysts degrades drastically over time, bulk metallic glass nanowires actually remain fairly level. While platinum can agglomerate and/or desorb from the carbon surface, it is already chemically bonded inside of the bulk metallic glass alloy. “That’s what makes this material so interesting and compelling,” says Taylor with a laugh.

As the first group to examine how bulk metallic glasses apply to fuel cells, the Taylor laboratory still has many exciting areas left to explore. In the immediate future, they are planning to refine the bulk metallic glasses used in fuel cells. Scientists already know which metals work well in supported catalysts, so ideally that knowledge can be incorporated into BMG design to maximize catalytic activity. Taylor is also investigating other BMG alloys that can prove useful for other fuel cell reactions. Practically, Taylor thinks fuel cells are a ways off from being a major part of the economy although he is more optimistic about developing micro fuel cells for a few niche areas. We may not be able to buy a fuel cell car just yet, but perhaps a fuel cell iPhone is not too far in the future.

Scanning electron microscope picture of one of the many patterns possible using flexible bulk metallic glass nanowires. Courtesy of Dr. Taylor.

About the Author
NANCY HUYNH is a junior MCDB major in Silliman College.

Acknowledgements
The author would like to extend her thanks to Dr. Andre Taylor for so helpfully explaining all these concepts during her first foray into chemical engineering and to his laboratory members for showing her around their facilities.